This invention relates to a method for patterning a photo resist film in which a photo resist is coated onto a semiconductor substrate to expose the photo resist coated thereafter to develop it, thereby to form a pattern on the photo resist film.
Heretofore, for forming a fine pattern on a semiconductor substrate, there has been conducted a procedure to coat a photo resist on the semiconductor substrate to apply baking thereto to carry out development after exposure or to conduct baking processing for a second time after exposure, thereafter to carry out development to form an etching mask, to therefore implement etching using the etching mask.
When such a conventional method is used, there arises the problem that solvent in a photo resist is volatilized according to the time that elapses since development processing after exposure, resulting in the variation of dissolving characteristic of the photo resist against developing solution and therefore in thickening line width of the photo resist.
In FIG. 1, change in the line width of the photo resist by the time elapsed since development processing after exposure is shown, wherein the abscissa shows the leaving time (elapsed time) and the ordinate shows line width of the photo resist. In this figure, circular marks indicate the points obtained by plotting the measured values of the line width of the photo resist when an experiment is conducted by changing the elapsed time and the graph K is an estimated curve showing the relationship between the elapsed time estimated from the measured result, and the line width of photo resist. From this graph K, it is seen that the line width of the photo resist becomes thick by about 0.95 (.mu.m) by leaving the photo resist for about two hours. This causes no problem in the case of continuously conducting a sequence of processings from the coating of photo resist to the development to form a pattern of the photo resist. However, in the case where respective processings are not conducted continuously, the line widths of photo resists on different semiconductor devices may differ from each other. Moreover, if baking processing is implemented after exposure, thickening of the line width of the photo resist may be further accelerated according to the type of photo resists.
In FIG. 2, there are shown changes in the line width of the photo resist according to the exposure time in the case where development is implemented immediately after exposure, and in the case where baking processing is implemented after exposure to conduct development, wherein the exposure time (m sec) is taken on the abscissa and the line width (.mu.m) of the photo resist is taken on the ordinate. The measured values of the line width of the photo resist when an experiment was conducted by changing the exposure time, are plotted with circular marks in the case of carrying out development immediately after exposure, and are plotted with triangular marks in the case of implementing baking processing for 60 seconds at a temperature of 110.degree. C. after exposure. The respective estimation curves M.sub.1 and M.sub.2 showing the relationships between the exposure time and the line width of the photo resist in the case of development being carried out immediately after exposure and in the case of implementing baking processing after exposure are plotted with solid lines, respectively. From these estimation curves M.sub.1 and M.sub.2, lowering of the sensitivity corresponding to about 30 to 40 (m sec) occurs in the photo resist. This brings about a lowering in the throughput corresponding to about 40 seconds per lot (24 semiconductor substrates) processing.
The photo resist used here is a photo resist including e.g. quinone diazide as a photosensitizer. The chemical structure of typical material, 1-, 2-naphthoquinonediazidesulfonyl is shown below. ##STR1##